1. Cross-Reference to Related Applications
This application claims the benefit of U.S. Provisional Application No. 60/019,529 filed Jun. 10, 1996, the contents of which are fully incorporated herein by reference.
2. Field of the Invention
The present invention relates to Aspergillus porphobilinogen synthases and isolated nucleic acid fragments comprising nucleic acid sequences encoding the porphobilinogen synthases. The invention also relates to nucleic acid constructs, vectors, and host cells comprising the nucleic acid sequences as well as methods for producing the porphobilinogen synthases.
3. Description of the Related Art
Heme, a chelate complex of protoporphyrin IX and iron, serves as a prosthetic group of hemoproteins. Protoporphyrin IX consists of a porphyrin ring, substituted with four methyl groups, two vinyl groups, and two propionic acid groups, which acquires an iron atom to form heme. The biosynthesis of heme from glycine and succinyl-CoA involves eight enzymatic steps. The second enzyme in the pathway is porphobilinogen synthase (also called aminolevulinic acid dehydratase) which catalyzes the condensation of two molecules of 5-aminolevulinic acid to form porphobilinogen. Porphobilinogen synthase is a rate-limiting enzyme in the heme biosynthesis pathways of Neurospora crassa and Saccharomyces cerevisiae.
The conversion of an apoprotein into a hemoprotein depends on the availability of heme provided by the heme biosynthetic pathway. The apoprotein form of the hemoprotein combines with heme to produce the active hemoprotein. The active hemoprotein acquires a conformation which makes the hemoprotein more stable than the apoprotein to proteolytic attack. If the amount of heme produced by a microorganism is less relative to the amount of the apoprotein produced, the apoprotein will accumulate and undergo proteolytic degradation lowering the yield of the active hemoprotein.
In order to overcome this problem, Jensen showed that the addition of heme or a heme-containing material to a fermentation medium led to a significant increase in the yield of a peroxidase produced by Aspergillus oryzae (WO 93/19195). While heme supplementation of a fermentation medium results in a significant improvement in the yield of a hemoprotein, it is non-kosher, costly, and difficult to implement on a large scale.
The cloning and expression of a porphobilinogen synthase gene from Saccharomyces cerevisiae (Labbe-Bois and Labbe, 1990, In, Dailey, H. A., ed., Biosynthesis of Heme and Chlorophylls, McGraw-Hill, Inc., New York, page 258) has been disclosed.
It is an object of the present invention to provide new porphobilinogen synthases and genes encoding same.